Logging while drilling (LWD) techniques for determining numerous borehole and formation characteristics are well known in oil drilling and production applications. Such logging techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, micro-resistivity, acoustic velocity, acoustic caliper, physical caliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range. Such LWD measurements (also referred to herein as formation evaluation measurements) may be used, for example, in making steering decisions for subsequent drilling of the borehole.
LWD sensors (also referred to herein as formation evaluation or FE sensors) are commonly used to measure physical properties of the formations through which a borehole traverses. Such sensors are typically deployed in a rotating section of the bottom hole assembly (BHA) whose rotational speed is substantially the same as the rotational speed of the drill string. LWD imaging and geo-steering applications commonly make use of focused FE sensors and the rotation (turning) of the BHA (and therefore the FE sensors) during drilling of the borehole. For example, in a common geo-steering application, a section of a borehole may be routed through a thin oil bearing layer (sometimes referred to in the art as a payzone). Due to the dips and faults that may occur in the various layers that make up the strata, the drill bit may sporadically exit the oil-bearing layer and enter nonproductive zones during drilling. In attempting to steer the drill bit back into the oil-bearing layer (or to prevent the drill bit from exiting the oil-bearing layer), an operator typically needs to know in which direction to turn the drill bit (e.g., up or down). Such information may be obtained, for example, from azimuthally sensitive measurements of the formation properties.
One drawback associated with the above described configuration (in which the FE sensors are rotationally coupled to the drill string) is that the vibration and shock sensitive FE sensors are subject to high lateral, axial, and torsional vibrations during normal drilling operations. Conventional FE sensor deployments are known to be susceptible to vibration and shock related errors and failures. Another drawback associated with the above-described conventional FE sensor deployments is that azimuthal logging techniques require a substantially uniform drill string rotation rate during drilling in order to suitably reduce statistical errors in the azimuthally focused logging data. While the above-mentioned conventional deployments are serviceable, and have been commercially utilized, an improved apparatus and method for acquiring near-bit formation evaluation sensor measurements is needed. In particular, there is a need for an apparatus that is less susceptible to shock and vibration related errors and failures and that is capable of providing both azimuthally focused and non-azimuthally focused formation evaluation sensor measurements.